On first sight I would agree, but...so, the part on second picture is sacrum? On first picture illiac fossa (the part, which is on third picture pointing up)? Or are these parts the pubic bones?(sorry, I don't know the english names)

To me,it does not seem, that you could join two such pieces together like vertebrates. Also, the part, which should probably be the spinous process seems to be expanded in the end, like if there were joint...

I sent it to a friend that works in the museum. This is what he wrote.

It is a mammal's occipital condyle - surrounding the entrance hole to the brain case. the broad collar of bone surrounding it (which is a fragment of a larger plate plate) and the large size suggest it is a young / juvenile whale. which species i wouldn't know, but bigger than a dolphin (too big for a young horse / cow, for example). in young mammals, the bones have not sutured, so fall to bits when disarticulated, and this is just a fragment of the larger occipital condyle plate.